Monitoring mesoplastic concentrations in estuarine waters: A case study in the River Guadalquivir (Southern Spain)

Rivers act as pathways transporting anthropogenic waste from inland sources to the sea, including large quantities of plastic. Estuaries are rich and diverse environments where the interaction between fluvial processes and the tidal regime results in complex dynamics that affects residence time and net transport plastics in the water column. The use of plankton nets (small mesh sizes, e.g. < 500 µm) to collect microplastic samples allows characterizing plastic particles of different sizes. At the same time, given the variability in plastic particles abundance, the limitation in number of samples and/or volume filtered per sample, may cause a large bias in such characterization. Generally, particle abundance decreases toward larger particle sizes, and therefore the collection of representative samples for a certain plastic size range depends on sampling effort. Here, we analyze abundance and weight of micro- (< 5mm) and mesoplastics (5-25 mm) collected in estuarine waters at the River Guadalquivir through bi-monthly monitoring over one year. Our results demonstrate that establishing ratios between micro- and meso-plastics concentrations can lead to large uncertainties when a limited number of samples is used in the analysis, causing strong bias in the extrapolation of mass budgets for mesoplastics. Collecting representative samples for mesoplastics implies a change in the current monitoring methods to specifically target such particle size range.